1. The lower altitudinal boundary of a snow-covered area, especially of one that is perennially covered, such as the snowcap of a mountain.
2. The fluctuating latitudinal boundaries around the polar regions marking the extent of snow cover.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.